The exemplary embodiments disclosed herein relate generally to traffic signals, and, more specifically, they relate to light emitting diode traffic signals.
The basic technology relating to light emitting diode (LED) traffic signals is well established and such traffic signals are in use worldwide. LED traffic signals present numerous advantages over common incandescent lamp traffic signals. Use of LEDs provides a power consumption savings and extremely long life in comparison to common incandescent light sources. The long life span creates improved reliability and sharply lowered maintenance costs.
LED signals have an extremely long service life that has increased with each new generation of LEDs. Incandescent lamps, while having a much shorter service life, have relatively constant light output until a total failure occurs, i.e., burnout of the light filament. LED signals, over an extended period, have gradually diminishing light output. Further, LED light output is negatively affected by temperature. In extreme climate or during unnaturally warm periods LED light output diminishes during the day and then returns to a normal level during cooler periods at night.
Thus, while LED traffic signal technology offers high reliability and low power consumption, it introduces complexity to the overall road traffic control system. Two of the most important issues that need to be addressed are interfacing and monitoring.
Thus, under the current standards, a signal state endangering traffic due to a “single failure” shall be prevented. If the first “single failure” is not apparent, the occurrence of an additional independent “single failure” shall be considered. A signal state endangering traffic due to the combination of both failures shall be prevented. If the first failure is detected by a manual proof test or an on-line test, the detection shall occur within the test proof interval specified by the manufacturer and the probability of a second failure which could cause an unsafe condition within this interval shall be less than 10−5 per year.
A “single failure” refers to any individual component failure. An “unsafe condition” refers, for example, to a situation where the traffic signal does not generate light when energized and the traffic controller does not detect the failure.
Presently, traffic controllers generally monitor the traffic signal input current to detect a failure. It is assumed that the measured input current always represents the output light. The traffic signal is equipped with an independent monitoring circuit that checks the light output and sets the traffic signal in high impedance state in case of a failure. However, if the traffic signal independent monitoring circuit becomes defective due to a faulty component, the traffic signal may continue to operate and the failure in the monitoring circuit is not apparent to the traffic controller and is not detected. In that situation, a subsequent traffic signal failure that can endanger the public is now possible because the independent monitoring circuit is defective or disabled.
The present invention contemplates a new and improved apparatus and method that resolves the above-referenced difficulties and others.